Suspended between the rocks

ARTICLE This elongated wooden house in Bohuslän is like no other – designed like a tied-arch bridge, it spans between two rocks to make minimal impact on nature. This makes its footprint less than one square metre.

On the island of Hamburgö in Bohuslän perches a long wooden house that almost seems to float above the rolling granite boulders. The villa was designed by Per Tamsen and Beate Hølmebakk of Norwegian practice Manthey Kula, informed entirely by the site conditions. The couple had a chance to buy the inaccessible plot four years ago, and immediately agreed that they wanted to make as little impact on nature as possible. Along with structural engineer Finn-Erik Nilsen, they developed this unusual house as a tied-arch bridge, with two glulam arches from which the house is suspended on posts.

»Every job and plot is different, and here we didn’t want any blasting or filling in such a beautiful setting. The house links the attractive outdoor spaces on the site and the location captures plenty of sun and views for the rooms inside,« explains Beate Hølmebakk.

The chosen solution influenced the wider design of the building.

»When we settled on this rocky plateau, we had a span of 29 metres and a house length of 25 metres. The building had to be shallow in order not to exceed the permitted development, « says Per Tamsen.

The load-bearing structure comprises two glulam arches that are anchored in the rock and also form a key element of the home’s visual appearance. Wooden posts have then been added, either hanging down or rising up, depending on where in the arch they are. The 1.8 metres between the posts is occupied by the windows.

»The roof trusses rest on the hanging posts and on the posts in the middle wall, except in three places where there are open spaces and we’ve used ties instead,« states Per Tamsen. To stabilise the building against the strong winds up on the hill, the roof and floor feature stiff panels on top of diagonal beams.

»The beams carry the forces out to each gable wall and into the ends of the glulam arches. In the middle of the wide house, where you come out onto the hillside via the dining area, steel ties secure the roof and floor beams to the outside decking and railings, which are anchored in the bedrock. This all combines into a construction that can withstand a ull-blown west coast storm,« says Finn-Erik Nilsen, who was responsible for the structural engineering.

The glulam arches are fixed to the rock with four pierced steel feet and 20 millimetre stainless steel rebar that has been drilled down and chemically anchored in place.

»One of the biggest challenges was measuring and making the fixing points in exactly the right place, so the prefabricated glulam could just be lifted onto them,« says Finn-Erik Nilsen.

The location up on the hill, with no direct link to the road, was another major challenge. When Patrick Edvardsson, foreman for local construction company JND Byggteknik which built the house, first saw the site, he realised they were going to need to build a temporary wooden work platform around the whole structure.

Architect Beate Hølmebakk

»The first thing we did was to buy a consignment of pulpwood, which we used as posts. Then we built a work platform onto them. We used a four-wheel drive with a petrol-driven winch to pull the material up a slipway that we made. The platform started 20 metres from the road, then we were able to lift the material onto it using a crane,« he says.

Some of the material was also lifted into place by helicopter, not least the glulam arches, which weighed 1400 kilos each. But because the helicopter could only lift a tonne, they were delivered in two 700 kilo sections.

»To be on the safe side, I bought a hanging scale and checked the first elements by suspending them from a crane,« adds Patrick Edvardsson.

The glulam arches are a prominent feature of the design, both externally and internally. From the outside, the arch is an attractive shape glimpsed through the façade, with the upper part of the front wall made entirely of glass. The lower part of the façade and the gable ends are clad in spruce from northern Sweden, and treated with iron sulphate. The underside of the house is shaped like a pitched roof, but upside down. This accommodates all the infrastructure – pipes and cables – while adding a sculptural note to the building.

It’s exciting when you get an idea where one thing leads to another and where the structure, architecture, light and space combine so boldly and become part of the landscape,« says Beate Hølmebakk.

And the house is just as striking inside. From end to end, there is an unbroken 25 metre view through the whole building. Combined with a ceiling height of 4.2 metres and sea views through the glazed walls, the overall impression is of incredible space and light. Per Tamsen and Beate Hølmebakk also designed the site-built desks, benches and kitchen cabinets for the house, so they can enjoy the panoramic views that, in the summer, are partially framed with trees and greenery.

One of the challenges was that the house sagged slightly during construction. Patrick Edvardsson thinks this is because the wood in the glulam arches was not completely dry to begin with.

»The question was whether to make the kitchen counter different heights or follow the floor. In the end, we decided to give the counter a slight incline that matches the slope of the floor. If you were to put a drop of water on the counter, it would probably run to the middle. But because we’re only talking about a couple of millimetres, it’s not obvious,« he says.

The four bedrooms are arranged in a row along the back of the house. When the sliding doors to the rooms are open, you get the same amazing sightlines through the house – and for privacy you just close them.

All the cupboards, drawers and doors were specially designed and custom-built for the house. Like the other internal surfaces, they are made of knot-free pine plywood. The only things painted are the pine floor, which is a high-gloss grey, and the window frames in the same grey.

»There are no exclusive material choices here – we’ve used pine throughout. The exclusivity lies in the attention to detail, « says Beate Hølmebakk.

Per Tamsen points up at where one of the ties runs into the wall.

»Because we couldn’t drill holes for the tie, we made an ornamental little triangle there. And because of that, we also fitted the power socket on the diagonal next to it. There’s a real joy in working on the details like this, and getting to design and construct at the same time».

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